This proposal describes an investigation of the feasibility of a system which will utilize ultrasonic radiation forces to selectively move and separate the cellular constituents of blood. It will be possible to develop a device based upon this phenomenon which can perform plasmapheresis within the hypodermic needle. In this device there will be no restriction of fluid flow, but the formed elements of the blood will be differentially acted upon by the generated forces which will expel or repel them from the needle. If this goal can be achieved, it will allow the withdrawal of plasma directly from the donor or patient leaving the cellular components unaffected in the blood stream. This device (a solid-state electronic instrument) will be technically much simpler than current centrifugal or filtration devices, its total extracorporeal blood volume will be less, it will not require anticoagulants, etc. to be returned to the patient, and it will not subject the cells and platelets to the traumas of centrifugation or filtration. We believe that the proposed device will find several applications. Because operating and disposable costs will be low, this device can replace the manual procedure used in the collection of source plasma for production of plasma derived products, and it will replace the centrifugal or filtraion devices used to provide large amounts of plasma for therapeutic plasma exchanges or removal of specific plasma agents which are being used as successful therapies for a large number of chronic diseases which have heretofore defined conventional therapy.